1. Field of the Invention
The present invention relates to a low-drop voltage regulator.
2. Background Discussion
Regulators are systems for automatically varying and maintaining a predetermined physical output quantity within a predetermined range despite variations in other disturbance quantities affecting the system.
Such systems typically involve conflicting requirements including those of compensating frequency while at the same time maintaining the accuracy of the system.
The rating parameters normally characterizing automatic regulating systems include:
a. Error. Defined as a variation in the output quantity in relation to a reference input quantity, due to numerous factors affecting mass production of the system, and which must be measured under steady conditions using a definite input quantity configuration.
b. Regulation. Defined as a variation in the output quantity due to variations in disturbance quantities and which must be measured under steady conditions using disturbance quantities varying within a definite range.
c. Settling time. The time taken to restore the output quantity to the correct value following a rapid variation in a disturbance quantity, and which must be measured using a disturbance quantity having a definite variation speed and amplitude.
d. Peak error. Defined as the maximum deviation of the output quantity from its normal operating value, in the presence of a rapid transient disturbance, specified herein as point c, and which must be measured under the same conditions as in point c.
A voltage regulator is a circuit for regulating the voltage applied to loads or user equipment absorbing a limited though not specifically defined amount of current. The load and the regulator are supplied with a supply voltage, and a reference voltage is also available, supplied by a supposedly accurate source, but with a poor current supply capacity. The reference voltage supplied to the regulator represents the quantity with which the output voltage is compared, and the disturbance for the voltage regulator substantially consists of the current supply to the load and the supply voltage, variations in both of which tend to affect the output voltage.
The current market demand is for voltage regulators with extremely good error and regulation characteristics. In fact, the increasingly widespread application of sophisticated control systems in traditionally exacting environments in terms of disturbance and reliability, as is the case with cars, has led to an increasing demand for electronic components conforming to increasingly strict requirements. More specifically, on the one hand, a desired system should be so designed as to prevent partial failure jeopardizing overall operation of the system, whereas a certain amount of degradation is normally acceptable. This desire requires dividing the voltage regulating function into various sections, so as to prevent failure in one which would the others. On the other hand, an increase in the sophistication of the system demands a similar increase in precision, as for example in the case of microprocessor systems involving digital/analog and analog/digital conversion, wherein the integrity of the numeric data depends on the ration of the analog data (ratiometry principle). At the very least, therefore, the regulated voltages must be practically identical, thereby also yielding the above demand for superior error and regulation characteristics.
For transferring power from the supply to the output, voltage regulators employ the power transistors, both N type (bipolar NPN or N-channel MOS) and P type.
Though the first type (featuring N type transistors) is subject to fewer problems of stability, the voltage drop in the power transistor poses limitations in applications in which the supply voltage is close in value to the output voltage.
The second type includes what is known as "low-drop" regulators, which operate satisfactorily even when the supply voltage is extremely close in value to the output voltage, but which present greater frequency stability problems as compared with the first type. To overcome this problem, the device must therefore be provided with a compensating capacitance. Due to the recent demand, however, for limiting radio interference, capacitive elements on regulated voltage lines must present an extremely low equivalent series resistance (ESR). For technical reasons, such capacitors are low value capacitors. Regulated voltage lines are fitted with higher-value capacitors for sustaining loads requiring high instantaneous current, and the ESR of which is necessarily high, particularly at very low temperatures, such as is required for automative applications. The conflicting demand for a high capacitance for improving frequency stability combined with a low ESR for limiting radio interference require trade-offs which inevitably satisfy neither requirement.
A typical prior art low-drop voltage regulator is shown in the circuit diagram of FIG. 1, wherein number 1 indicates a known regulator having an input terminal 2 connected to a supply voltage 3 of value Va; and an output terminal 4 connected to a load 5. Regulator 1 comprises a P-type power transistor 6, in this case a bipolar PNP transistor, having the emitter connected to input terminal 2, and the collector connected to output terminal 4. The base of power transistor 6 is driven by an error comparator, consisting of a current-output, low-voltage-gain, operational amplifier 10, via a high-input-impedance drive transistor 11 and a resistor 12. More specifically, operational amplifier 10 has its non-inverting input connected to a voltage source 13 supplying reference voltage V.sub.R and its inverting input connected to output terminal 4. The output of operational amplifier 10 is connected to the base of drive transistor 11, here represented by a bipolar NPN transistor, but generally consisting of more complex (e.g. Darlington) configurations for increasing input impedance. The collector of drive transistor 11 is connected to the base of power transistor 6, while the emitter is grounded (reference potential line) via resistor 12. For frequency stability reasons, an impedance 15 of value Z.sub.c is provided between the output of operational amplifier 10 and ground and between output terminal 4 and ground. Provision is made for a capacitor 16 which, requiring a value of at least 10 .mu.F, must be electrolytic. Unfortunately, the above capacitors present a significantly high ESR, which increases with a fall in temperature, and which, are represented symbolically in FIG. 1 by resistor 17. Such capacitors 16 negatively affect the frequency stability of regulator 1, which is thus limited to other than very low temperature applications.
In FIG. 1, an error voltage V.sub.e is present between in inputs of operational amplifier 10, and represents the difference between reference voltage V.sub.R and output voltage V.sub.u. If V.sub.c is the output voltage and g.sub.m is the transconductance of operational amplifier 10, voltage V.sub.c can be derived by the following formula: EQU V.sub.c =V.sub.e *g.sub.m *Z.sub.c,
thus giving a voltage gain of operational amplifier 10 of g.sub.m *Z.sub.c. Under normal (d.c.) operating conditions, the gain of operational amplifier 10 is generally relatively low, ranging from 100 to 500. Indeed, for frequency stability reasons, gain must necessarily be low and impedance Z.sub.c presents a capacitive frequency compensating using a capacitor of limited value (more specifically, integratable), the capacitor, which is located between the output of operational amplifier 10 and ground or the supply line, is connected between the base and collector of a transistor for amplifying its capacitance. For all of its effectiveness, such a technique is fairly empirical, in that the value Z.sub.c of known devices cannot be expressed in the form of an analytical function straightforward enough to enable the use of automatic control theories.
Output voltage V.sub.c is supplied to the base of high-input-impedance drive transistor 11 and, via resistor 12, is converted into the current I.sub.b designated by the following formula: I.sub.b =V.sub.c /R, where "R is the resistance of resistor 12, which current, from the base of power transistor 6, is multiplied by gain B of transistor 6 to give output current I.sub.u designated by the following formula: EQU I.sub.u =B*I.sub.b.
A quantitative estimate of the error and regulation characteristics of the known regulator in FIG. 1 can be made as follows. Assuming, as is normally the case, a value of 5 V for V.sub.R and V.sub.u, when I.sub.u varies from a minimum value of O A to a maximum value which need not be defined, the base current I.sub.b of power transistor 6, which is directly proportional to the output current, also switches from a minimum (O A) to a maximum. To maximize efficiency of the capacitive component Z.sub.c of impedance 15, for achieving effective frequency compensation and compactness (small integration area), resistance R of resistor 12 must be maximized as described below, and such that its maximum voltage, corresponding to maximum current I.sub.b, is as high as possible, compatible with operation of drive transistor 11 and supply voltage V.sub.a, supply voltage V.sub.a reaches the required minimum value V.sub.a(min) where, EQU V.sub.a(min) =V.sub.u +V.sub.ce6(sat)
where V.sub.ce6(set) is the voltage between the collector and emitter of power transistor 6 when saturated.
Under such conditions, V.sub.c, which is normally expressible as follows: EQU V.sub.c =V.sub.bell -V.sub.cell -V.sub.be6 +V.sub.a
where V.sub.bell and V.sub.cell are respectively the base-emitter and collector-emitter voltage drop of drive transistor 11, and V.sub.be6 is the base-emitter voltage drop of power transistor 6, presents a maximum possible value V.sub.c(max) where, EQU V.sub.c(max) =V.sub.bell -V.sub.cell(sat) -V.sub.be6 +V.sub.a(min)
or, EQU V.sub.c(max) =V.sub.bell -V.sub.cell(sat) -V.sub.be6 +V.sub.u +V.sub.ce6(sat)
where V.sub.cell(sat) is the collector-emitter voltage drop of transistor 11 when saturated.
Roughly, V.sub.bell =V.sub.be6, and V.sub.cell(sat) =V.sub.ce6(sat) yielding 1. EQU V.sub.c(max) =V.sub.u =5 V.
When V.sub.c =O and I.sub.b =O, V.sub.c is within a range of 5 V, and V.sub.e, which is supplied to operational amplifier 10, is within a range of 5 V/500=10 mV or 5 V/100=50 mV (depending on whether the gain of operational amplifier 10 is 500 or 100, respectively.
The need for maximizing resistance R of resistor 12 is explained as follows. Approximately the impedance Z.sub.c with its capacitive component C, yields Z.sub.c =1/sC, so that the transfer function F, the input and output of which are respectively represented by the current from operational amplifier 10 and current I.sub.b, equals 1/sCR. Since this function depends on the product of R and C, for a given transfer function, to minimize C and so reduce the size of capacitor C (as required for integrated applications), R must be maximized.
Known regulators of the aforementioned type therefore provide for load and line regulation ranging from 10 mV to 50 mV, which fails to conform with current requirements in terms of precision.
The same failure also applies to the error characteristic. In fact, all the "errors" generated downstream from operational amplifier 10 are supplied to its input and divided by the relatively low gain of the amplifier. In the case of base current I.sub.b, for example, this may vary substantially, up to 100%, due to mass production spread, which variation, divided by g.sub.m, becomes the variation in the voltage V.sub.e required for error correction. Being independent of external variables, such as V.sub.a and I.sub.u, this variation voltage may even be as high as 10 mV, which is added to the various regulation components mentioned above for determining the total difference between required voltage V.sub.R and actual voltage V.sub.u.
It is, therefore, a general object of the present invention to provide a low-drop voltage regulator having improved error, regulation and speed performance characteristics, and which provides for frequency stability using a low-value, low-ESR, radiofrequency output capacitor.